This invention relates to the field of module bays, specifically directed air flow in module bays commonly used in semiconductor processing.
FIG. 1 is a schematic diagram of a conventional semiconductor processing apparatus, as seen from a top view. In the middle, there is a relatively open main bay for wafer transport, in which a strong downward flow of cleanroom air is maintained at all times. This flow sweeps any contaminant (e.g., heat, chemicals, aerosol particles) down to the floor, through which they are removed to the fab exhaust. This flow has a secondary purpose as well, which is to supply a purge flow for the vertically stacked module bays next to the main bay. Both of these processes serve to prevent contaminant transport between modules on different sides of the main bay.
FIGS. 2(a,b) are schematic diagrams of a conventional configuration of vertically stacked module bays (e.g., B2) and the air flow in a main bay, as seen from perspective (FIG. 2a) and cut-away (FIG. 2b) views. The main bay and the module bays are separated by a solid wall W2, which has slit orifices (e.g., S2) connecting the module bays to the main bay. The flow used to purge the module bays is drawn through these slit orifices. For typical flow conditions, the average horizontal flow speed through each slit orifice is smaller than the vertical flow speed in the main bay.
FIGS. 3(a,b,c) are schematic diagrams of a generic of the interior of a conventional module bay, as seen from several views. The module bay is a rectangular box within which a roughly cylindrical processing module is centrally placed. The processing module is composed of a base, a lid, a mechanism for raising and lowering the lid from the base (not shown), and fairly open support structures connecting the base to the module bay floor (not shown). Various chemical transport and electrical lines are also connected to the processing module. When a chemical or thermal process is performed for a wafer within the processing module, the processing module is sealed. An internal purge is used to remove contaminant from the processing module interior, but some contaminant may remain at the time the processing module is opened to unload the processed wafer and load the next wafer for processing. The module bay purge flow is intended to remove any contaminant from the processing module by sweeping it to the back of the module bay and exhausting it before any significant fraction can be transported out to the main bay during wafer unloading/loading.
In one conventional processing apparatus, there can be 20 vertically stacked module bays, containing vapor-prime, vacuum-bake, and chill-plate processing modules. The processing apparatus can also contain other processing modules such as spin-coaters and developers. Table 1 shows representative parameters, for air at 20.degree. C. and 1 atm., for a conventional processing apparatus.
TABLE 1 Quantity Symbol Value or Range Kinematic viscosity .upsilon. 0.01 ft.sup.2 /min Contaminant diffusivity D &lt;0.02 ft.sup.2 /min Purge flow rate V' 2-4 ft.sup.3 /min External downward velocity U.sub.E 60 ft/min Module bay width L.sub.0 16 in. Module bay length L.sub.1 18 in. Module bay height H.sub.0 7 in. Lid bottom to base top H.sub.1 0.875 in. Slit length S.sub.0 9 in. Slit width W.sub.0 1:1 in. Module diameter D.sub.0 10 in. Exhaust area A 1 in..sup.2 Slit face velocity U.sub.0 = V'/(S.sub.0 W.sub.0) 29-58 ft/min Module bay face velocity U.sub.M = V'/(L.sub.0 H.sub.0) 2.5-5.0 ft/min Contamination decay length L.sub.M = (D/U.sub.M)ln(10) &lt;0.1-0.2 in. Purge change-out time t.sub.p = (H.sub.0 L.sub.0 L.sub.1)/V' 0.3-0.6 min Reynolds number of jet Re = (W.sub.0 U.sub.0)/.upsilon. = V'/(S.sub.0 .upsilon.) 270-540
The module bays are configured in several columns with several module bays in each column. Each module bay is a rectangular box within which a roughly cylindrical processing module is placed (adjacent module bays can have different processing modules within them). These processing modules typically have a lid and a base and are closed during processing but open for wafer loading and unloading. An operational cycle occurs as follows: the processing module is opened, a robot arm carrying a wafer passes through the slit from the main bay into the module bay and loads the wafer into the processing module, the robot arm is withdrawn back into the main bay, the processing module is closed, the process is performed within the processing module (which may include some purging within the closed processing module), the processing module is opened, the robot arm enters the module bay and unloads the wafer from the processing module, and the robot arm is withdrawn back through the slit. There is also the possibility of lateral transfer of wafers between certain adjacent module bays by dedicated internal robot arms.
Many of the processes involve chemicals or levels of heat which could adversely impact other processes if transported to the vicinity of these processes. The spin-coaters and developers are especially sensitive to these types of contamination. To maintain process quality, it can be important to ensure that contaminant (chemical or heat) transport from the vertically stacked module bays to the spin-coaters and developers remains below prescribed levels, which are generally much more stringent than corresponding environmental, safety, and health requirements. Several strategies are typically employed to minimize contaminant transport. First, a continual downward flow of clean air is maintained at all times in the main bay. Contamination entering the main bay is thus swept downward and exhausted before reaching the sensitive processing modules. Second, transport between the vertically stacked module bays and the main bay is minimized by minimizing the contact areas, namely the slits through which the robot arm passes from the main bay into the module bays to load and unload wafers. Any contamination entering the main bay from the module bays passes through these slits. Third, purge flows are maintained from the main bay into the module bays at all times. These purge flows are intended to sweep any contamination to the back of the module bays, from which it is exhausted. The purge flow rate per module bay (shown in Table 1) in conventional practice can be approximately 4 ft.sup.3 /min, which yields a total purge flow rate (for 20 module bays) of 80 ft.sup.3 /min.
The contemporary cost of cleanroom air used to purge the module bays, including pre-use treatment to condition it for cleanroom applications and post-use treatment to render it benign for release to the environment, has been estimated to be roughly $2-$4 per CFM per year on average. The costs of certain waste streams such as acid exhaust can be considerably higher. Thus, reducing air-use requirements by operating changes and by improved designs is highly desirable both for current tools and for next-generation tools. This is particularly important for the semiconductor industry as emphasis shifts to larger wafers. A reduction in cleanroom air use directly reduces the cost of equipment operation. It also permits more equipment to be operated with the same total amount of air use, so additional equipment can be brought on-line without having to increase a fab's air-handling capability. Limitations on total air use may be mandated by law in the future for environmental, safety, or health reasons.
In present configurations of module bays with slit orifices, there is no device for controlling the direction of the flow entering the module bay from the main bay. This lack of control causes the purging to be far from optimal and susceptible to system perturbations. For example, an increase or decrease of the downward flow speed in the main bay will affect the angle at which the purge flow enters the module bay through the slit orifice. Similarly, a change in volume flow rate through the slit orifice will also change the entry angle. Such a change in entry angle will alter the purging of the module bay and (more importantly) of the processing module interior in an uncontrolled manner. Accordingly, there is a need for improved module bays and associated processing apparatus that effectively and predictably purge contaminants while requiring reduced cleanroom air flow.